This invention relates to a magnetic bubble memory device, more particularly a magnetic bubble memory device operated by a bias magnetic field perpendicular to the magnetic memory chip and a rotating magnetic field parallel to the chip.
Generally, a magnetic bubble memory device is constituted by a magnetic film, a magnetic bubble chip comprising a pattern of a soft ferromagnetic film formed on the magnetic film, lead wires for transmitting signals between the chip and an external circuit, a coil block for applying a horizontal rotating magnetic field to the chip and a magnet block for appyling a perpendicular bias magnetic field to the chip. Any desired information is written, stored and read out from the memory device by forming, transporting and dividing magnetic bubbles in the pattern of the soft ferromagnetic film.
As will be described later with reference to the accompanying drawing, in a typical magnetic bubble memory device, the magnetic bubble chip is mounted on an insulating substrate at substantially the center thereof and a fine output signal line is connected between the chip and a lead pin on one end of the insulating substrate. The length of the electric wire including a detector installed in the magnetic bubble chip is of the order of microns, whereas the length of the signal line between the chip and the lead pin is of the order of centimeters which is much longer than the internal wiring. On the other hand, coils for applying a horizontal rotating magnetic field to the magnetic bubble chip are wound about the insulating substrate so as to surround the chip and the signal line, and a voltage of several tens of volts is induced across the coil for producing the rotating magnetic field by a high frequency AC current flowing through the coil. This terminal voltage induces an induction noise in the wiring inside the chip and in the signal line through the capacitance between the coil and the wiring inside the chip and between the coil and the signal line. Especially, since the length of the signal line is of the order of centimeters whereas that of the wire in the chip is of the order of microns so that the former occupies a larger area of the insulating substrate than the latter, the induction noise induced in the signal line is very large. In other words, the coils utilized to generate the rotating magnetic field constitute a source of a large external disturbance for the signal line when the currents flow through the coils. To decrease the induction noise in the signal line, it has been proposed to install a dummy or reference detector in the magnetic bubble chip and the dummy detector and the main detector were operated differentially. However, its noise decreasing effect was not sufficient.